The use of dispersive type electrodes in connection with electrosurgical or body stimulation apparatus is well known and these electrodes are used as current paths or grounding pads to provide conducting paths between the patient and the stimulation or electrosurgical equipment. While these electrodes can be used for any skin conducting application where currents may be high enough to cause heating only the electrosurgical application will be described for simplicity.
At present, disposable electrosurgical dispersive electrodes divide into two general groups, those electrodes which provide a conductive interface with the body of a patient and those electrodes which provide a capacitive interface with a patient's body.
The conductive dispersive electrodes are more generally used and comprise a metal plate held in contact with a patient's skin, often with a jelly-like electrolyte or by use of a conductive adhesive which is disposed between the metallic electrode and the skin of the patient. The problem with such conductive electrodes is that generally in use there is a nonuniform temperature distribution of the skin immediately under or near the electrode for the reason that the current tends to concentrate in that skin area having the least resistance path which is generally at the edge of the electrode nearest the active electrode surgical site.
The other type of electrosurgical dispersive electrode is the capacitive type which is provided with a plastic film or like insulating area separating the conductive metal plate from the patient's skin. This arrangement provides a large impedance and a voltage drop of a capacitive nature between the skin surface and the metal electrode. When attached to completely clean, uniform skin, this type of electrode results in more equal distribution of current over the entire area of the electrode thus reducing uneven temperatures or hot spots in certain areas of the patient's skin. When such dispersive electrodes are properly applied that they are in intimate contact with the patient's skin, the large voltage drop across the dielectric film is out of phase with the current through the film resulting in practically no heat generation within the dielectric layer itself. However, considering hair and other skin irregularities it is exceedingly difficult in actual practice to provide for absolute intimate contact between the dielectric film of the capacitive dispersive electrode and the patient's skin.
The Geddes et al Pat. No. 4,387,714 discloses a hybrid type of electrosurgical dispersive electrode in which a layer of conductive adhesive is placed between the dielectric layer of a capacitive ground pad and the patient's skin. This arrangement clearly improves the intimacy of contact between the electrode and the patient's skin. However, it has been found in practical use that the layer of conductive adhesive below the dielectric layer prevents uniform current dispersion provided by a capacitive dispersive electrode. When all of the elements of the patient's skin surface are tied together with a continuous sheet of conductive adhesive, the freedom of each element of the patient's skin to see the high impedance current source which is provided by the corresponding element of the dielectric capacitor action is lost. Instead, the result is similar to connecting a capacitor in series with a normal conductive type dispersive electrode. Nonuniform heating of the tissue under and near the described hybrid dispersive electrode is the result, very similar to that obtained with the usual conductive dispersive electrode.